1. Field of the Invention
The present invention relates generally to coatings of insulative and ablative materials. More particularly, the present invention relates to ablative coatings on exposed components of aerospace structures and a method of forming such coatings.
2. State of the Art
Ablative coatings are thermally insulating coatings typically utilized as heat shields for exposed surfaces of aerospace equipment such as rockets, missiles, space shuttles and similar vehicles. The ablative coating serves to protect the structure from high thermal energy experienced due to high velocity conditions, for example during launch or re-entry into the earth's atmosphere. The ablation of such a coating is a known phenomenon by which energy incident upon an ablating material is dissipated through vaporization of the material rather than conversion of the energy into heat. Thus, during exposure to the heat energy, the material of the ablative coating is eroded away through vaporization, dissipating the incident heat energy by converting the solid material into vaporous matter.
Formation of ablative coatings of aerospace structures is conventionally accomplished through one or more processes known in the art. In one process, the ablative coating is formed by molding the ablative material into a sheet and subsequently applying the sheet to a surface of the aerospace structure with an adhesive. Such a process is very labor intensive and time consuming. Molded sheets of ablative material, while exhibiting a certain amount of flexibility, are difficult to shape and conform to the complex shapes found in aerospace structures. Thus, a considerable amount of time is spent cutting, shaping and otherwise manipulating the sheet material in order to cover a desired surface. Additionally, much of the ablative material may be wasted in trimming and shaping the sheet material.
In some applications, adhering the sheet material to the surface requires the structure to be placed in a vacuum bag or a similar process in order to accomplish proper adherence of the sheet material to the surface of the structure. This practice helps to minimize and hopefully eliminate voids between the sheet of ablative material and the surface of the aerospace structure. Additionally, once a surface is covered with the sheet material, substantial work is required to achieve a desired surface finish, which may entail machining, grinding or sanding. Such post-application surface work is also required with regard to joints formed at adjacent edges of abutting sheets of ablative material.
Hand troweling is another conventional technique which has been utilized to apply ablative material to the surface of an aerospace structure. Hand troweling includes applying the ablative material to the surface of the aerospace structure by a hand trowel and allowing the ablative material to cure in place. This technique allows for the ablative material to be placed directly on the structure and also provides a means of coating complex shapes. However, hand troweling requires a high degree of skill and is very time consuming and labor intensive. Additionally, substantial rework is still required to produce the desired surface finish and shape. Also, it is difficult to monitor and control the thickness of the applied coating, particularly if the coating is placed over complex surfaces and the thickness is to be varied from one area of the aerospace structure to another.
Yet another conventional technique of applying ablative coatings is to spray the coating directly onto the structure. Similar to the technique of hand troweling, spraying the ablative coating allows the material to cure in place. Also, spraying lends itself to coating aerospace structures having complex geometries. However, spraying typically requires the use of an ablative composition having a relatively low viscosity in order to pass the ablative material through the spraying equipment. Due to the nature of spraying, including the lower viscosity material, only a nominal thickness of the material may be applied to the aerospace component at one time. In order to achieve a sprayed-on coating with any substantial thickness, multiple coatings must be applied. This becomes an extremely time-consuming process, wherein a coating is applied and then cured to a specified level prior to application of a subsequent coating.
Additionally, spraying does not provide adequate thickness control. Spraying is particularly deficient in applying ablative coatings which require a varied thickness over the surface of the aerospace component. Thickness control, in part, becomes a process of reworking the coating by hand after it is cured. Indeed, in some cases, surface work may be required between spray coatings, adding to the time and labor required to achieve a satisfactory coating.
The technique of spraying also results in incidental overspray. Overspray results in material waste and also requires the use and maintenance of special facilities, such as a spray booth, again increasing the cost of applying the ablative coating. Furthermore, ablative coatings often include a fibrous or particulate material in the ablative composition which poses additional problems for spraying. Fibers and particulates can clog the spraying equipment, requiring excessive cleaning and undue maintenance.
As an example of the time and labor involved with spraying an ablative coating onto structures having complex shapes, FIGS. 1A-1C show an aerospace component at various stages during spray application of an ablative coating. Referring to FIG. 1A, a ring strap 10 utilized in conjunction with an aeroskirt structure of a rocket is shown prior to application of an ablative coating. The ring strap 10 includes a number of nut plates 12 which protrude from an arcuately shaped plate 14. Such a configuration makes it difficult to provide an ablative coating of consistent thickness. FIG. 1B shows the ring strap 10 with an ablative coating 16 applied by multiple sprayed coatings of ablative material. The ablative coating 16 is not conformal to the original geometry of the ring strap 10 and generally results in peaks 18 and valleys 20 formed in the surface of the ablative coating 16. However, design requirements often specify a more conformal coating with a surface finish which is improved over that of a sprayed coating. Thus, as seen in FIG. 1C, substantial post-application rework is required to produce a substantially conformal coating 16′. Such rework may involve trimming, grinding, machining, sanding and the like. A substantial amount of time is involved in, first, building up the ablative coating 16 (FIG. 1B), and, second, reworking the ablative material to produce a conformal coating 16′ (FIG. 1C) with a satisfactory surface finish. Additionally, it is evident that a great deal of material is wasted in coating a part such as the ring strap 10 in such a manner.
Furthermore, with each of the conventional techniques discussed above, variations exist in the resultant coatings from one component to another like component. For example, tight tolerances are difficult to maintain consistently regarding thickness, surface finish, density and other material characteristics. The ability to consistently apply coatings with repeatable and predictable results has been somewhat elusive with such techniques. Unacceptable variations are due, in part, to the manner in which the material is applied and, in part, to the fact that operators of different skill levels may be applying the coating.
In view of the shortcomings in the art, it would be advantageous to provide a method of applying ablative insulating coatings to structures in a reduced amount of time and with less wasted material. Additionally, it would be advantageous to provide a method which provides repeatable and predictable results.
In providing repeatable results, the method would desirably minimize the opportunity for variance by reducing the number of variables affected by operators forming the ablative coating. Additionally, it would be desirable to provide a more standardized process such that variance would not be realized in the application of the material by one operator when compared to application by another operator.
Such a method would also desirably result in improved surface finish and thickness control, thus minimizing the amount of touch-up work required.